Dynamic Image Reconstruction in Nuclear Medicine

1. Dynamic Image Reconstructionin Nuclear Medicine Ryan O’Flaherty Kyle Fontaine Krystal Kerney

2. Acquisition techniques • Static • Data acquisition starts after the radiotracer is distributed and settled in the targeted tissues. • Static SPECT provides one static 3D image of the distribution of the radiotracer. • Dynamic • Data acquisition starts immediately after the injection of radiotracer. • Dynamic SPECT provides a series of 3D images. Each image represents the distribution of the radiotracer at a certain time. • Dynamic images convey information about tracer movement through different body tissues.

3. Data Acquisition • Input • 3D volume (body injected by the radiotracer). • —Process • Recording the activity of tracer in the 3D volume. • Output • Aset of 2D projections taken from different angles (Sinogram).

4. Image Reconstruction • Input • Aset of 2D projections taken from different angles (Sinogram). • Process • Reconstructing the 3D volume back from the recorded projections. • Output • 3D volume (radiotracer activity in the body tissues). • Time Activity Curves (TACs) can be extracted from the reconstructed time-dependent images for tissues in interest. • Example of Myocardium Time Activity Curves

5. Bsplines • Instead of reconstructing a time independent-volume, we try to estimate the input functions of tissues in interest. • Time basis functions (B-splines) represent the temporal behavior of radioactive tracer in the imaged tissues.

6. Task • Our task • Generate desired time activity curves based off the time basis functions (Bsplines) that we create. • Generate Bsplines Create TAC’s  Cluster TAC’s • Example TAC to mimic:

7. Mathematical formulation • Similarly to static image reconstruction, a form of the equation below is used: • However, the case of dynamic imaging requires that the imagined volume V_k be split up into several imagined volumes, each representing the volume at a given time, m. • C is an algorithmically determined coefficient and f represents the time dependent operator, the B-spline

8. Mathematical formulation • Thus, the new equation for the sinogram which will be used to reconstruct V_k is below: • Where: • P is the sinogram vector • S is the system matrix • n is the number of pixels • m is the number of projections (and thus represents time) • K is the number of voxels

9. Coefficient Calculation * From Mamoud’s Presentation • This objective function x^2 is to be minimized based on the parameters C and f. This allows for the verification of the coefficients

10. results • An example of our time activity curves before clustering • These were extracted from our reconstructed volumes (unable to view) • If we could, it would be 72 volumes concatenated with each other Relative Activity (Arbitrary Units) Time (s)

11. Results • Shown below - the Bsplines and their corresponding TAC’s Relative Activity (Arbitrary Units) Time (s)

12. results Relative Activity (Arbitrary Units) Time (s)

13. results Relative Activity (Arbitrary Units) Time (s)

14. results Relative Activity (Arbitrary Units) Time (s)

15. New Results Relative Activity (Arbitrary Units) Time (s)

16. New results Relative Activity (Arbitrary Units) Time (s)

17. New results Relative Activity (Arbitrary Units) Time (s)

18. New results Relative Activity (Arbitrary Units) Time (s)

19. Best result Our result Goal Relative Activity (Arbitrary Units) Time (s)

20. Refining Results • Take output clustered TAC  Feed back into C code as our spline input • Attempt to initialize the algorithm using different initial conditions • Hope to find new, more accurate, local minimum • Won’t necessarily ‘refine’ (can even have opposite effect)

21. Refined Results Refined Previous Best Result Relative Activity (Arbitrary Units) Relative Activity (Arbitrary Units) Time (s) Time (s) Goal

22. Refined results Refined Result Previous Result Relative Activity (Arbitrary Units) Relative Activity (Arbitrary Units) Time (s) Time (s) Goal

23. Refined Results Previous Result Refined Result Relative Activity (Arbitrary Units) Relative Activity (Arbitrary Units) Time (s) Time (s) Goal

24. Summary • Our task was to understand the concepts behind dynamic imaging, and then reproduce given results. • The team was able to recreate the given data using event ques from a supplied sinogram. These events are translated into knots, which represent the beginning, end, and inflection points in a B-spline curve. • Each of these curves were used as operators to affect algorithmically determined constant coefficients. These constant/curve pairs are summed to represent Time Activity Curves, which denote the temporal dependence of specific radiotracers within known tissues of a patient. • Attempted to initialize the algorithm with different initial conditions (clustered TAC outputs) in hopes for more accurate minimization.

25. Summary • Our success was based on varying the B-splines we input, and noting the influence of the changes on the clustered TAC’s. • Notes • Open ended B-spline necessary to avoid symmetrical TAC’s • Such tendencies don’t make sense when considering the temporal activity of radiotracer in tissue • Clustering B-splines in either direction (more B-splines in the beginning, or more B-splines later) caused irregular TAC’s • Reducing the number of B-splines gave us TAC’s that more closely mimicked our desired output • However, increasing the number of B-splines didn’t necessarily cause irregular TAC outputs. • Result of refining • Small but noticeable refinements for all 3 attempts • More experimentation required (very time consuming)

26. review of journal article (Part 1) • Article title: The role of nuclear imaging in the failing heart: myocardial blood flow, sympathetic innervation, and future applications • Journal: Heart Failure Reviews • Year: 2010

27. Nuclear imaging in the failing heart • Heart failure affects approximately 5 million patients in the United States!

28. SPECT/pet imaging in the failing heart • Nuclear imaging is the only modality with sufficient sensitivity to assess blood flow and innervation of the failing heart. • Innervation is excitation of the heart by nerve cells. • SPECT is most commonly used for evaulation of myocardial perfusion (blood flow to the heart). • PET allows for quantification of myocardial blood flow. • Both can be used for evaluation of diagnosis, treatment options, and prognosis in heart failure patients.

29. Sympathetic innervation of the heart • Sympathetic innervation (excitation) represents another important parameter in patients with heart failure. • Sympathetic nerve imaging with 123-iodine. metaiodobenzylguanidine (123-I MIBG) is often used for assessment of cardiac innervation. • Abnormal innervation is associated with increased mortality and morbidity rates in patients with heart failure. • 123-I MIBG can be used to categorize patients by risk for ventricular arrhythmias or sudden cardiac death.

30. Potential of nuclear imaging in heart failure • Detailed information on several biological processes in heart failure • Myocardial blood flow • Sympathetic innervation of the myocardium • Myocardial perfusion imaging represents the mainstay of cardiovascular radionuclide applications • Sympathetic innervation imaging is increasingly used in patients with heart failure

31. Myocardial blood flow in the failing heart • SPECT • Well-established and safe imaging modality for the evaluation of location, extent and severity of myocardial perfusion defects. • 3 commercially available SPECT tracers: 201Thallium, 99mTc-tetrofosmin, and 99mTc-sestamibi. • 201Thallium • 99mTc-tetrofosmin • 99mTc-sestamibi • PET • Several PET tracers currently available for assessment of myocardial perfusion • 2 approved for clinical use by the FDA • N-13 Ammonia (13NH3) • Rubidium-82 (82Rb) • Both can be used for absolute quantification of myocardial blood flow

32. Table of SPECT/PET tracers

33. Dynamic imaging and tracer kinetics • Dynamic imaging with multiple time frames requires a high count density and advanced data processing • For tracer kinetic analysis, arterial input function and myocardial kinetics are measured from regions of interest in dynamic images • Absolute flow quantification is achieved by employing compartmental modeling analysis to the obtained time-activity curves. • Various tracer kinetic models have been established according to the nature of each PET tracer

34. review of journal article (Part 2) • Last time: • Introduction • Potential of nuclear imaging in heart failure • Myocardial blood flow in the failing heart. • Dynamic images and time-activity-curves • Today: • Sympathetic innervation in the failing heart • Using SPECT • Using PET

35. Sympathetic innervation in the failing heart • The sympathetic nervous system can do two things: •  strength of contraction •  amount of blood returned • Sympathetic input: • speeds SA depolarization (HR • decreases AV delay (HR) • increases contractility in contractile cells (SV) • In sum: sympathetic input increases heart rate and degree of contraction per beat

36. Regulation of the sympathetic nervous system

37. Imaging with SPECT/Planar • Radionuclide imaging of the norepinepherine analog metaiodobenzylguanidine (MIGB) baleled with 123-iodine (123-I). • Planar and SPECT imaging are performed in the early and late phase of the 123-I MIBG protocol. • Planar images from Left-Anterior oblique view and provide information on global sympathetic innervation pattern. • SPECT images are used to assess regional abnormalities in cardiac sympathetic innervation.

38. Imaging with SPECT/Planar

39. Imaging with PET • In contrast to SPECT/Planar imaging, PET maps the sympathetic nervous system with superior temporal and spatial resolution. • Spatial resolution of 4-7mm. • Temporal resolution allows for development of dynamic images which can be used to assess tracer kinetics. • Can be used to quantify the absolute amount of tracer and its time-dependent kinetics.

40. Two categories of PET tracers • Radiolabeledcatecholamines • Molecularly identical to endogenous neurotransmitters • Undergo similar uptake, release and metabolic pathways • Radiolabeled catecholamine analogs • False neurotransmitters • Follow the same uptake and release mechanisms without being metabolized like endogenous transmitters • Example: Hydroxyephedrine labeled with carbon-11

41. Hydroxyephedrine labeled with carbon-11 • One of the most frequently applied PET tracers for cardiac sympathetic nerve imaging as it shows high affinity for a common uptake mechanism. • Can be used for accurate assessment of regional neuronal defects as it has been shown to distribute equally within the myocardium in physiologic conditions.

42. More uses for pet imaging • Cardiac innervation has also been explored in heart failure patients who underwent cardiac transplantation. • PET has also been used to evaluate the relation between cardiac sympathetic innervation and ventricular arrhythmias.

43. Future role of nuclear imaging • In the future scientists hope to: • Use nuclear medicine for prevention of overt heart failure. • To develop molecular-targeted imaging techniques that will provide further insight into the pathophysiology of the failing heart

44. MICAD • The Molecular Imaging and Contrast Agent Database (MICAD) is an online source of scientific information regarding molecular imaging and contrast agents (under development, in clinical trials or commercially available for medical applications) that have in vivo data (animal or human) published in peer-reviewed scientific journals.

45. micad • MICAD is a key component of the “Molecular Libraries and Imaging” program of the National Institutes of Health (NIH) Common Fund, designed to accelerate medical research for disease detection, diagnosis and therapy. By linking programs in molecular imaging, molecular probes, and molecular libraries, the NIH Common Fund provides much needed support for the development of new, more specific therapies for a wide range of diseases such as cancers, Alzheimer’s and Parkinson's diseases.

46. micad • MICAD is edited by a team of scientific editors and curators who are based at the National Library of Medicine, NIH, in Bethesda, Maryland. • The database includes, but is not limited to, agents developed for PET, SPECT, MRI, ultrasound,CT, optical imaging, planar radiography, and planar gamma imaging. The information on each agent is summarized in a book chapter format containing several sections such as Background, Synthesis, in vitro studies, Animal Studies, Human Studies, and References.

47. Micad • From http://www.ncbi.nlm.nih.gov/books/NBK5330/ you can download a CSV file of all of the imaging agents. • I did this and trimmed the database down to only PET and SPECT agents that were related to the heart.